System and method for control of layer formation in an aluminum electrolysis cell

ABSTRACT

And objective of the present invention is to provide an improved method and system for use for control of layer formation in an aluminium electrolysis cell and exploitation of the heat. The present invention attains the above-described objective by a reordering of the fundamental structure of a Hall-Héroult cell, providing two alternatives, eliminating the need for thermal insulation. In a first embodiment the ordering from the inside to the outside is: Electrolyte-sidelining-heat tubes-steel shell. In a second embodiment the ordering from the inside to the outside is: Electrolyte-sidelining-steel shell-heat tubes.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to heat regulation in general and particularly improved method and system for cooling over a large area, suitable for use for control of layer formation over an extended area in an aluminium electrolysis cell and exploitation of heat.

2. Background Art

During production of aluminium with electrolysis technology of today based on so called Hall-Héroult cells, the operations of the cells depend on the formation and maintenance of a protective layer of frozen electrolyte in the side walls of the cell. This frozen bath is called side layer and protects the side lining of the cells against chemical and mechanical wear, and is an essential condition for achieving long lifetime of the cells. The frozen bath operates simultaneously as a buffer for the cell with regards of changes in the heat balance. During operations the heat generation and the heat balance of the cell will vary due to unwanted disturbances of the operation (changes in bath acidity, changes in alumina concentration, changes in interpolar distances, etc.) and desired activities on the cells (metal tapping, change of anode, fire, etc.). This causes the thickness of the layer of the periphery of the cell to change and in some cases the layer will disappear entirely in parts of the periphery. Then the side lining will be exposed against the electrolyte and metal, which in combination with oxidizing gasses will lead to corrosion of the side lining materials causing these to erode. During operations over time run outs in the side can result from such repeated occurrences. It is therefore of importance to control formation of layer and layer stability in Hall-Héroult cells. For Hall-Héroult cells with high current densities model calculations show that it will be difficult to maintain the side layer of the cell due to large heat generation. For such cells and for traditional cells with heat balance problems it will therefore be a condition for a long life cell that one is able to maintain the layer protecting the side lining.

During production of aluminium in accordance with Hall-Héroult principle, this takes place at present with relatively high use of energy as measured in kilo watt hours per kilo aluminium. The heat generation of the electrolysis cells takes place as a result of ohmic voltage drops in the cell, for instance in current feeds, produced metal and particularly in the electrolyte. Approximately 55% of input energy to the electrolysis cell is used for heat generation in the cell. Data from literature indicates that approximately 40% of the total heat loss from the cells is lost through the side lining. Due to the high heat loss and the protecting frozen layer in the side lining it is a preferable place to place elements for heat regeneration in this area of the cell.

There is a desire for optimizing control of layer formation and heat regeneration. In order to optimize both of this purpose at the same time it is important that heat regeneration takes place as close to the formed side layer as possible. This will lead to the control of and speed on layer formation is as fast as possible, and that temperature difference between input and output cooling medium is as large as possible. The latter is preferable for exploitation/regeneration of energy.

Furthermore, due to the large scale of electrolysis cells, it is also desirable to control said layer formation over an extended area since loss of layer formation over a small area can be damaging.

The traditional method of removing heat was to use air convection over the entire surface area of the cell, resulting in limited potential for exploitation of the removed heat.

From the known art one should refer to granted patent NO 318012, corresponding to WO/2004/083489. This describes a sidelining formed with hollows for flow-through of a cooling medium. The manufacturing process of this, however, is complex and requires the side linings to be moulded with hollows formed preferably before the material is sintered.

From the known art one should also refer to patent application NO 20101321, brought into the PCT-phase as PCT/NO2011/000263, of the present applicant. This describes a system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining provided with at least one hollow for heat transfer and at least one heat tube, characterized in that the heat tube is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining. The manufacturing process of this, however, is complex and requires providing the side linings with a large number of heat tubes, typically heat pipes, along the surface of the sidelining, each requiring separate cooling. An insulation layer is provided between the sidelining and an outer steel shell.

The problems with the above system are that it requires specifically formed sidelining, that it cannot be retrofitted without interrupting the process in the cell, and that on overheating the heat tubes are destroyed with limited possibilities of repair or retrofitting.

There is therefore a need for a method and a system overcoming the above mentioned problems.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

A main objective of the present invention is to provide an improved method and system for use for control of layer formation in an aluminium electrolysis cell and exploitation of the heat, with a simplified and flexible structure.

Means for Solving the Problems

The objective is achieved according to the invention by a system for use for control of layer formation in an aluminium electrolysis cell as defined in the preamble of claim 1, having the features of the characterising portion of claim 1 and a method for control of layer formation in an aluminium electrolysis cell as defined in the preamble of claim 8, having the features of the characterising portion of claim 8.

The present invention attains the above-described objective by a reordering of the fundamental structure of a Hall-Héroult cell. Whereas the traditional ordering from the inside to the outside is:

Electrolyte-sidelining-heat tubes-insulation-steel shell,

the present invention provides two alternatives.

In a first embodiment the ordering from the inside to the outside is:

Electrolyte-sidelining-heat tubes-steel shell

Optionally this embodiment is further provided with insulation outside the steel shell.

In a second embodiment the ordering from the inside to the outside is:

Electrolyte-sidelining-steel shell-heat tubes

Optionally this embodiment is further provided with insulation outside the heat tubes.

In a first embodiment a system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising a sidelining block, a shell, and a heat tube is provided, wherein the sidelining block is in thermal contact with the shell, and that the heat tube is operable to remove heat from at least one of the sidelining and the shell.

In one embodiment the ordering the ordering from the inside to the outside is sidelining block-heat tube-steel shell

In a second embodiment the ordering the ordering from the inside to the outside is sidelining block-steel shell-heat tube

In a preferred embodiment the heat tube is a heat pipe.

In another embodiment the heat tube is a thermosyphon.

In a preferred embodiment the at least one of thermal paste and thermal conductive glue is applied between sidelining block and shell.

In a preferred embodiment the a thermal insulation layer is provided as the outermost layer.

In a second embodiment a method for control of layer formation in an aluminium electrolysis cell said electrolysis shell comprising a sidelining block, a shell and a heat tube, wherein the sidelining block is in thermal contact with the shell, and that the heat tube is operable to remove heat from at least one of the sidelining and the shell is provided, conducting the heat away using said surface attached heat tube.

Effects of the Invention

The technical differences over the traditional ordering is that the need for thermal insulation no longer is necessary when using heat tubes

These effects provide in turn several further advantageous effects:

-   -   it makes it possible to simplify the structure,     -   removing the thermal insulation saves space and cost,     -   the steel shell can be kept at a low temperature     -   keeping external surfaces at a low temperature improves safety     -   there is more freedom in designing the cell     -   adding thermal insulation as the outermost layer improves heat         absorption by the heat tubes even further

The two embodiments that are envisaged have different advantages.

For the first embodiment the thermal contact between the sidelining and the steel shell is less important since the cooling takes place in the sidelining.

The second embodiment has the advantage over prior art as well as the first embodiment in that

-   -   there is no significant difference in thermal coefficient of         expansion between the SiC and the steel shell,     -   the heat tube can be made from a material having the same         thermal expansion and conduction properties as the steel shell,     -   there is no need for purpose formed blocks with respect to the         heat tubes,     -   the heat tubes can be manufactured more easily and cheaper than         compared with prior art,     -   heat tubes can be maintained, repaired and replaced during         operations,     -   the steel shell does not have to be penetrated by heat tubes,         and     -   it can be retrofitted to an existing cell without interruption         to the process

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings, wherein:

FIG. 1 shows state of the art of a Hall-Héroult cell in the form of a sidelining block, and a steel shell or casing,

FIG. 2 shows a detail section of the embodiment of FIG. 1 together with section as seen from the side,

FIG. 3 shows state of the art of a Hall-Héroult cell in the form of a sidelining block with hollows provided with heat tube, and a steel shell or casing,

FIG. 4 shows a detail section of a first embodiment,

FIG. 5 shows a detail section of a second embodiment

DESCRIPTION OF THE REFERENCE SIGNS

The following reference numbers and signs refer to the drawings:

1 Anode hanger 2 Anode carbon block 3 Liquid electrolyte 4 Liquid aluminium 5 Cathode carbon 6 Frozen electrolyte 7 Insulating brickwork 8 Steel shell 9 Ramming paste 10 Heat insulation 11 Sidelining block 12 Heat tube 13 Condensation unit for heat tube 14 Condensation fins 15 Thermal paste between sidelining block and steel shell

DETAILED DESCRIPTION

The invention will in the following be described in more details with references to the drawings showing embodiments

PRINCIPLES FORMING THE BASIS OF THE INVENTION

A basis for the invention is the realisation that with cooling using heat tubes 12 the sidelining component ordering is more freely changed than for a traditional cell design as shown in FIG. 1 with details are shown in FIG. 2, and even a state of the art cell using active cooling as known from previously mentioned prior art and shown in FIG. 3, and that the thermal insulation also becomes an optional feature.

On removing the thermal insulation from its normal position between the sidelining block 11 and the steel shell 8, the sidelining block and the steel shell comes in close contact, wherein the steel shell is heated by radiation and by conduction. Improved thermal contact by conduction can be achieved by applying thermal paste or thermal conductive glue between sidelining block and steel shell.

BEST MODES OF CARRYING OUT THE INVENTION

The most preferred embodiment of the system according to the invention shown in FIG. 5, comprises an electrolyte bath enclosed by a sidelining block 11 of silicon carbide further enclosed by a steel shell 8. The sidelining block is and the steel shell is in close thermal contact by applying a thermal paste or thermal conductive glue between sidelining block and steel shell. Outside the steel shell one or more heat tubes 12 are attached. Investigations show that the thermal resistance between the electrolyte and the steel shell is sufficiently low to maintain the side layer in the frozen state. Investigations also show that the heat tubes should be attached with great care so that the heat tubes remain in good contact with the steel shell 8 in order to avoid hot spots or differential thermal expansions.

A second preferred embodiment of the system according to the invention shown in FIG. 4, comprises an electrolyte bath enclosed by a sidelining block 11 of silicon carbide further enclosed by a steel shell 8. The sidelining block is and the steel shell may be in close thermal contact by applying a thermal paste or thermal conductive glue between sidelining block and steel shell. Inside the steel shell and at the sidelining block one or more heat tubes 12 are attached. The heat tubes can be thermally attached to the sidelining blocks in several ways, a preferred method is described in the aforementioned PCT/NO2011/000263, however not using the insulating layer as disclosed therein.

In the case of failure of the heat tubes 12 normal operations of the cell can be achieved without interruption by removing any outer layer of thermal insulation applied outside the heat tubes. For high reliability operations it is preferred that this insulation is removed or detached quickly and before the side layer has become liquid. A hinged system for plates of thermal insulation is envisaged, whereby the plates swing open on overheating. Opening can be trigged in several ways, including electromechanically and by a bi-metallic arm. The opening motion can likewise be executed in several ways, including but not limited to gravity action, spring loading, hydraulics or pneumatic opening.

ALTERNATIVE EMBODIMENTS

A number of variations on the above can be envisaged. For instance the steel shell 8 and the heat tubes 12 might not be as separate parts, rather the heat tubes 12 could be moulded into the steel shell during manufacture, as a monolithic unit.

Also an outer layer of thermal insulation can be applied outside the heat tubes. This results in lower losses of heat and thus more heat into the heat tubes and lowering the external surfaces even further. It should however be noted that this is an optional feature and that the invention will operate without this thermal insulation. In the case of this embodiment it should be noted that were the heat tubes or the cooling of the condensation units 13 of the heat tubes to fail, the temperature of the heat tubes would rise and possibly reach the same temperature as the electrolyte and the pressure inside the heat tubes would increase while the metal of the heat tube would lose its strength until resulting in failure. By removing the thermal insulation the heat tube temperature can be maintained at around 500° C. and thus failure will be avoided. In this mode the cell would operate as a conventional cell, without heat tubes.

While the shell 8 of the cell is described as being made of steel, it should be clear that any other material will also work as long as it can conduct heat and withstand the temperatures involved.

INDUSTRIAL APPLICABILITY

The invention according to the application finds use in control of layer formation in an aluminium electrolysis cell and exploitation of the heat. 

1.-8. (canceled)
 9. A system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising: a sidelining block (11); a shell (8); and a heat tube (12), wherein the sidelining block (11) is in thermal contact with the shell (8), and that the heat tube (12) is operable to remove heat from at least one of the sidelining and the shell.
 10. The system according to claim 9, wherein ordering the ordering from the inside to the outside is sidelining block (11)-heat tube (12)-steel shell (8).
 11. The system according to claim 9, wherein ordering the ordering from the inside to the outside is sidelining block (11)-steel shell (8)-heat tube (12).
 12. The system according to claim 9, wherein the heat tube (12) is a heat pipe.
 13. The system according to claim 9, wherein the heat tube (12) is a thermosyphon.
 14. The system according to claim 9, wherein at least one of thermal paste and thermal conductive glue is applied between sidelining block (11) and shell.
 15. The system according to claim 9, further comprising a thermal insulation layer as the outermost layer.
 16. The system according to claim 10, wherein the heat tube (12) is a heat pipe.
 17. The system according to claim 10, wherein the heat tube (12) is a thermosyphon.
 18. The system according to claim 10, wherein at least one of thermal paste and thermal conductive glue is applied between sidelining block (11) and shell.
 19. The system according to claim 10, further comprising a thermal insulation layer as the outermost layer.
 20. The system according to claim 11, wherein the heat tube (12) is a heat pipe.
 21. The system according to claim 11, wherein the heat tube (12) is a thermosyphon.
 22. The system according to claim 11, wherein at least one of thermal paste and thermal conductive glue is applied between sidelining block (11) and shell.
 23. The system according to claim 11, further comprising a thermal insulation layer as the outermost layer.
 24. The system according to claim 12, wherein the heat tube (12) is a heat pipe.
 25. The system according to claim 12, wherein the heat tube (12) is a thermosyphon.
 26. The system according to claim 12, wherein at least one of thermal paste and thermal conductive glue is applied between sidelining block (11) and shell.
 27. The system according to claim 12, further comprising a thermal insulation layer as the outermost layer.
 28. A method for control of layer formation in an aluminium electrolysis cell said electrolysis shell comprising a sidelining block (11), a shell (8) and a heat tube (12), wherein the sidelining block is in thermal contact with the shell, and that the heat tube is operable to remove heat from at least one of the sidelining and the shell, characterized in conducting the heat away using said surface attached heat tube. 